It is known in the art to impregnate fiber structures with polymer. The prior art is replete with many published processes for the fabrication of reinforced fiber structures, and the impregnation of the fibers to coat them with polymer. Because the fiber roving or strand consists of hundreds and thousands of fine fibers of micron or sub-micron diameter, the major challenge for these processes is to ensure that the fibers are properly wetted by the polymer during fabrication so that air bubbles are not trapped in the composite. Improper wetting or infiltration of polymer into the fiber roving causes air bubbles to be trapped which can be retained in the fiber reinforced polymer composite. Consequently, the composite properties are seriously compromised. For example, subsequent use of such a fiber reinforced polymer composite as a feedstock for injection molding of a polymer composite product results in the expansion of the air bubbles retained in the composite during processing. This causes many voids to form in the product. Thus, a fiber reinforced product with inferior or unacceptable properties is formed.
Processes aimed at obtaining a well-coated, resin-coated fiber bundle have included processes which employ cylindrical pins or lobes along with fiber tension to spread apart the filaments and promote resin impregnation of the fiber bundle in a molten resin die. U.S. Pat. No. 4,439,387 to Hawley discloses a method of producing fiber composite material by using such a process. Hawley discloses a method of manufacturing a composite reinforcing structure by extruding a mass of fluid thermoplastic resin material in a flowable state in a stationary die while introducing a plurality of continuous lengths of reinforcing fiber strands into the die in the presence of the flowing mass to contact and coat each fiber strand. The process disclosed in Hawley is characterized by some degree of intermixing of thermoplastic material and the rovings are introduced into the coating channel almost on a perpendicular position, which places rupturing forces onto the roving fibers.
Another process enhances the impregnation of the fibers by alternating convex and concave pins in a molten resin bath or a die. U.S. Pat. No. 4,728,387 to Hilakos describes an assembly for the impregnation of a continuous length of fibers including a sequence of convex and non-convex surfaces over which the length of fibers is drawn under tension. A complete and homogeneous impregnation is obtained via the pressure of impingement on the surfaces which alternately separates and consolidates the fibers in sequence during their impregnation with the resin. Disadvantageously however, the impingement increases the probability of damaging the fibers as the spreading of the fibers is done through the use of mating surfaces touching the fibers. However, in this process there is the risk that a coated fiber will have its coating damaged.
U.S. Pat. No. 6,251,206 to Saito et al., discloses a method for spreading and resin-impregnation to produce continuous fiber-reinforced thermoplastic resin composite material. A reinforcing fiber bundle is spread by passing the fiber bundle through opening pins in a zigzag arrangement and simultaneously impregnating them with molten resin, which permeates into the spaces between the spread-out fibers. This process uses a fixed-type pin with an auxiliary pin and impregnation is effected via an impregnation box through which an impregnation resin flows while retaining a predetermined amount of the molten resin inside the box. The use of an impregnation box would not allow to spray the resin and is not as efficient as a rotating nozzle.
U.S. Pat. No. 5,091,255 to Hsu et al. discloses elongated granules of reinforcing fibers that extend generally parallel to each other and are longitudinally dispersed uniformly throughout a latex binder composition. The method of manufacturing such granules as disclosed in Hsu includes the steps of continuously passing reinforcing fibers through one or more baths of an aqueous, film-forming latex binder composition, to impregnate the filaments.
U.S. published patent application No. 2002/0180095 to Berard discloses the extrusion of a thermally conductive polymer composition containing a continuous core of carbon fiber reinforcement created in a machine configured to hold a spool containing a continuous strand of carbon fiber core material. In Berard's method, there is physical contact of the fibers with a solid surface during the coating process, as the carbon fiber strand is unrolled off the spool and is fed into a preheating chamber and then fed into a port in an extruding head on a pressure extruding machine. A molten polymer matrix is also fed into the extruding head thereby extruding the polymer matrix onto, around and between the individual carbon fibers contained in the strand.
U.S. Pat. No. 6,270,851 Lee et al. discloses an apparatus for preparing a resin coated fiber composite providing a narrow flow path for the fiber filaments in a zigzag shaped tunnel, and a plurality of resin inlet ports disposed along the tunnel to fill molten resin in the zigzag shaped tunnel, and pressurize the flow path of the fiber filamentous. Lee et al. provides a continuously tensioning of the fiber filaments and contacts between the fiber and the semicircular rings and so does not facilitate limited physical contact between the fibers and the solid surface during the coating process so as to lessen the rupturing stresses along the rovings.
U.S. Pat. No. 4,559,262 to Cogswell et al. discloses a fiber-reinforcing structure with exceptionally high stiffness is produced by wetting the reinforcing filaments with molten thermoplastic polymer in a continuous process separating the rovings into the individual constituent fibers by applying electrostatic charges to the rovings, prior to their entry into the molten thermoplastic polymer, or spreading the roving by passing it under tension over the outside of heated spreader surfaces while in the bulk of the impregnation bath. Such tension may cause damage to the structural integrity of the stretched fiber and especially to the coating of a coated fiber.
Conventionally, all of the fabrication processes try to overcome the bubble entrapment problem by physically pressing the fiber roving against a concave surface in order to spread the fibers while allowing molten polymer to infiltrate between the fibers. However, the major drawback with this solution is that most fibers are forced to rub against a static solid surface in order to spread the roving. This action will unavoidably cause at least some physical damage to the fibers. Moreover, in the case where the fibers are coated, this process is very likely to strip off a significant proportion of the coating from the fiber surface. Fibers whose coating has been stripped in this manner are poor candidates for certain applications of the final composite material such as electromagnetic interference shielding.
In view of the foregoing, there is a demand for a means of fabricating polymer-coated, metal-coated fiber materials without causing the metal-coated fibers to keep physical contact with a solid surface. In addition, there is a demand for a method of, and an apparatus for, producing continuous polymer-coated, metal-coated fibers wherein frictional forces (which might cause mechanical damage to the fibers or the stripping off of a coating, if present) between the fibers and solid surfaces is eliminated. There is also a demand for a means of fabricating polymer-coated fibers (especially for fibers with a coating) wherein the fibers do not have physical contact with the sprayer solid surface while being sprayed with polymer.